This invention generally relates to an apparatus for monitoring density of a gas inside a pressurized container. More particularly, the invention concerns apparatus for monitoring density of gaseous sulfur SF.sub.6 hexafluoride in switch gears and switching devices for use with high voltage electrical current.
In gas inclosed and insulated electrical switch gear for high voltage current, sulfur hexafluoride is sometimes used in place of air due to its high dielectric strength in comparison to that of atmospheric air. When sulfur hexafluoride is used, both its density and its pressure are important parameters that must be monitored: density being important due to its relationship to dielectric strength or insulating power of the gas; and pressure being important as a measure of the quenching capability of the gas. Accordingly, it becomes necessary to install monitoring devices in electrical switch gear for both density and pressure.
Typically, gaseous sulfur hexafluoride is contained in a closed pressurized system, in part due to its expense. Accordingly, changes in ambient temperature can be reflected in pressure variations within the system. Due to the classical relationship between density, pressure, and temperature under well known laws of the physics of gases, a temperature increase in a closed system is accompanied by a corresponding pressure increase. Thus, in the event a leak should occur during a period of increasing temperature, a pressure responsive gauge would not indicate the presence of the leak. in a closed system, gas density would drop immediately when a leak occurs and would thereby signal the presence of a leak.
Systems for monitoring gas density in a pressurized container are currently available. For example, one system utilizes a cylinder having a piston which acts on a spring-loaded ram. The spring-loaded ram has an end provided with a shoulder and includes an internal bore which accommodates a second loose ram. The second loose ram passes through the shoulder and has a neck beyond which the loose ram also extends. Between the shoulder of the spring-loaded ram and the neck of the loose ram are a plurality of bimetallic disks designed in such a manner as to equalize the pressure change attributable to a change in temperature. Thus, pressure-induced movement of the ram caused by a temperature change in the system is eliminated and resulting movement of the ram is a measurement of density. Such a density monitor is used particularly with sulfur hexafluoride in gas-insulated and enclosed switch gear and represents a specially manufactured system. Typically, the ram arrangement provides an electrical indication of the existence of a leak.